To engage in sexual intercourse with someone of the opposite sex in a manner which can only be described as 'hard' or 'rough.' The more injuries sustained, the greater the degree of ruining. The word is commonly used by males describing thier sexual conquests. To merc a girl, is similar to ruining a girl, but to ruin a girl means that the hardest sex she has ever had would be the one who ruined her. Can also mean to take a girl's vaginity, but in a selfish manner.